1. Field of the Invention
This invention relates generally to methods for determining the alkali metal content of liquids, and more specifically to an improved spectrophotometric determination of potassium and sodium cations in protein-containing liquids such as blood serum or plasma.
2. Background Art
Various clinical procedures have evolved over the years for determining alkali metals, notably sodium and potassium, in liquids and particularly in biological fluids such as blood serum. Such procedures are frequently used in monitoring renal functions, as well as being used to detect indications of other diseases. For example, in healthy human beings, the potassium level in the blood serum falls within a narrow range, generally between 3 and 6 millimoles per liter of blood serum. A potassium concentration of only 1.5 millimoles per liter can be fatal, resulting in respiratory depression and cardiac arrhythmia. Because the range of normal potassium level is limited and further because a relatively small deviation from the normal range can be devastating, clinical procedures for determining alkali metal levels must be highly accurate.
Conventional methods for determining the alkali metal content, and specifically sodium and potassium contents, of such fluids as blood serum have been used clinically. Typical of such techniques are flame photometry and ion-specific potentiometry.
One improvement in techniques for determining alkali metals was described by Sumiyoshi and Nakahara in Talanta, 24 (1977), 763. In the technique described, potassium was determined photometrically by first forming a complex between potassium and a monocyclic crown ether. The complex was then extracted into benzene as an ion pair with an anionic dye. Thus, the color intensity of the dye was a relatively accurate measure of the potassium content of the solution. Various other similar colorimetric methods have also been described in the literature. See, for example, Takagi, et al., Analytica Chimica Acta, 126 (1981), 185. and Charlton, et al., Clin. Chem., 28/9 (1982), 1857.
One of the shortcomings of colorimetric determinations of the prior art, including that disclosed in the aforedescribed Sumiyoshi, et al. article, is that all known methodologies require deproteinization of the serum or plasma being assayed prior to formation of the alkali metal-cryptand complex. The necessity for treatment to remove proteins can affect the alkali metal content of the serum or plasma and thus introduce inaccuracies to the procedure.